Im using four 74HC595 in parallel. I am shifting out using a modified shiftOut function. My query is about how consistent the registers latch is.

For some reason my whenever the latch is set to HIGH the last pin at least is going HIGH for a split second even though I have not told it to. (I only tell it to be HIGH at the very end before it loops)

By the way im only using pins output 1-36 and 40 from the shift registers. Is it ok for 37-39 to be 'floating'?

The term 'floating' only applies to input pins, not output pins unless the device has 'tri-state' output modes.And yes, it's ok not to have anything wired to unneeded output pins of the shift register.